Many applications which operate at a low voltage such as 3.3V, or 5V or 12V, benefit from being connected to a mains power supply. Previously, a conventional way of obtaining a low voltage was to use a high-voltage capacitive divider, in which a capacitor is used as an impedance. The capacitor current is rectified and used. This method is efficient, but the capacitor is large and expensive.
Another conventional solution is to use rectification and a switched mode DC-DC converter, which may be configured to operate with a high efficiency, but these are generally expensive, due to the high voltages involved. At the low duty cycles involved, these are challenging to control; for example to convert 325V down to 3.3V, the DC-DC converter would have to operate at only 1% duty-cycle.
A further known solution, which is generally less expensive than use of a DC-DC converter, is to use a linear voltage regulator, such as a low drop-out voltage regulator (LDO). A voltage regulator typically comprises a power FET and a differential amplifier. The differential amplifier compares the output voltage—or alternatively and more commonly a well-defined fraction of the output voltage—with a reference voltage, and drives the power FET in linear mode to maintain a fixed output voltage. A voltage regulator would be extremely inefficient if used to down-convert a mains voltage to a typical low-voltage: for instance, used with the same 220V mains supply resulting in peak voltages of 325V, to provide power for a 3.3V output voltage, it has to drop 322 V. So, in known solutions, the supply to the voltage regulator is normally provided by a capacitor, which is charged to an intermediate voltage (just above the output DC voltage for good efficiency) from the rectified mains. Establishing the intermediate voltage is typically done by so-called “gated rectification”, in which the mains is rectified by a bridge rectifier, and the output switchedly connected to the capacitor. It has been proposed to replace the voltage regulator with a DC-DC converter, such as a switched mode power converter.
However, ripple from the DC-DC converter may generate EMI.